Here's a link to a Johnathan Morrison YouTube demo of a Asus 31.5 4K display hooked up to a nMac Pro.
Even on a 31.5" display at full 4K resolution, text and the UI is way too small to be useful. But you can scale to the current iMac 27" 2560X1440 and if your graphics application can do it it will allow you to view the graphic window in full 4K at the same time.
Games? I'm not sure how that will work but probably something similar.
I've got a feeling that a 4 or 5K iMac will be a separate product and not the new standard large screen iMac. $$$. Who knows.
https://www.youtube.com/watch?v=KUvphmy0v2k
Even on a 31.5" display at full 4K resolution, text and the UI is way too small to be useful. But you can scale to the current iMac 27" 2560X1440 and if your graphics application can do it it will allow you to view the graphic window in full 4K at the same time.
Games? I'm not sure how that will work but probably something similar.
I've got a feeling that a 4 or 5K iMac will be a separate product and not the new standard large screen iMac. $$$. Who knows.
https://www.youtube.com/watch?v=KUvphmy0v2k